WhatsApp introduced end-to-end encryption by default across its communications in 2016.
WhatsApp completed its broad end-to-end-encryption rollout on April 5, 2016, after working with Open Whisper Systems, the organization associated with the Signal Protocol. The change covered messages, group chats, photos, videos, documents, voice calls, and other forms of communication supported by the service. In an end-to-end-encrypted conversation, the content is protected so that only the communicating users' devices are intended to read or hear it.
The 2016 announcement completed a process that had begun in late 2014, when WhatsApp started incorporating the Signal encryption protocol. This history is why some accounts describe encryption as being introduced in 2014 and others identify 2016 as the key date: partial implementation came first, while protection for every form of communication arrived later.
Telegram and Signal are also encryption-related messaging apps, but neither is the answer to this question. Facebook Messenger historically offered end-to-end encryption through optional Secret Conversations rather than making it universal at the time. WhatsApp's encrypted backups were introduced separately and initially required users to enable the feature.